From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id EC692C43334 for ; Thu, 2 Jun 2022 18:18:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237910AbiFBSSW (ORCPT ); Thu, 2 Jun 2022 14:18:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43434 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234209AbiFBSSU (ORCPT ); Thu, 2 Jun 2022 14:18:20 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 7C56C4CD4E for ; Thu, 2 Jun 2022 11:18:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1654193898; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8cILwmtkvSQjYb8QuEETKqwmhc+FPzA1S+q/o/FgcNc=; b=bo+AQDk7U0j+EkdfW8iNZyrXS0HBDrLudcS22oxAHzEHBYTu8qFx0nvP5fKnW94UDFYaX4 iV76PTF6o4MJr5wBEKcuEqCtL8ij0vlzs8jNyvdA2+nBAsBAF5L41HjbpaESmMJFn87Hw/ HpktZLEPYp3qbd/jQDlPfxbdmJmIi/g= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-148-paQpCGJ5NQWI91QC9NROAg-1; Thu, 02 Jun 2022 14:18:15 -0400 X-MC-Unique: paQpCGJ5NQWI91QC9NROAg-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 0548C801228; Thu, 2 Jun 2022 18:18:15 +0000 (UTC) Received: from [10.22.32.147] (unknown [10.22.32.147]) by smtp.corp.redhat.com (Postfix) with ESMTP id B090D1731B; Thu, 2 Jun 2022 18:18:14 +0000 (UTC) Message-ID: Date: Thu, 2 Jun 2022 14:18:14 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat_flush() Content-Language: en-US To: Tejun Heo Cc: Jens Axboe , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Ming Lei References: <20220601211824.89626-1-longman@redhat.com> <20220602133543.128088-4-longman@redhat.com> <42da456d-8f6a-3af0-4cd3-d33a07e3b81e@redhat.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/2/22 13:46, Tejun Heo wrote: > On Thu, Jun 02, 2022 at 01:26:10PM -0400, Waiman Long wrote: >> On 6/2/22 12:58, Tejun Heo wrote: >>> Hello, >>> >>> On Thu, Jun 02, 2022 at 09:35:43AM -0400, Waiman Long wrote: >>>> @@ -2011,9 +2076,16 @@ void blk_cgroup_bio_start(struct bio *bio) >>>> } >>>> bis->cur.ios[rwd]++; >>>> + if (!READ_ONCE(bis->lnode.next)) { >>>> + struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu); >>>> + >>>> + llist_add(&bis->lnode, lhead); >>>> + percpu_ref_get(&bis->blkg->refcnt); >>> Hmm... what guarantees that more than one threads race here? llist assumes >>> that there's a single writer for a given llist_node and the ref count would >>> be off too, right? >> The llist_add() function is atomic. It calls into llist_add_batch() in >> lib/llist.c which uses cmpxchg() to make the change. There is a non-atomic >> version __llist_add() which may be problematic in this case. Note that irq >> is disabled in the u64_stats_update* critical section, there shouldn't be a >> racing thread running in the same cpu. Other cpus will modify their own >> version of lhead. Perhaps the non-atomic version can be used here as well. > Ah, right, this is per-cpu, so there can be no second writer trying to add > the same node at the same time. Can you add a comment explaining the overall > design / behavior? Other than that, please feel free to add > > Acked-by: Tejun Heo > > Thanks. OK, I will send another patch to document the design in block/blk-cgroup.c. I don't want to touch this patch unless I need to correct some code here. Thanks, Longman >